GaAs-based quantum-well laser diodes each having an InGaAs strained quantum-well active layer or layers, especially ridge waveguide semiconductor laser diodes, have been intensively researched and developed as feasible higher-output power semiconductor laser diodes or light sources in optical devices for use in wavelength division multiplexing (WDM) systems. With the development of WDM systems, it is desired to further increase the optical output power of the ridge waveguide semiconductor laser diode.
In addition, the GaAs based ridge waveguide semiconductor laser diode attracts attention as a light source for use in an erbium-doped fiber amplifier (EDFA).
Referring to FIG. 1, a typical GaAs-based quantum-well laser diode will be described as an example. A layer structure 10 of a conventional ridge waveguide GaAs laser diode includes, for instance, a buffer layer 14 having GaAs or AlGaAs based compound semiconductor layer, a lower cladding layer 16, a first optical confinement layer 18, a second optical confinement layer 20, a first strained quantum-well layer 22, a barrier layer 24, a second strained quantum-well layer 26, a third optical confinement layer 28, a fourth optical confinement layer 30, a first upper cladding layer 32, an etch stop layer 34, a second upper cladding layer 36, and a cap layer 38 consecutively formed on a n-GaAs substrate 12. The second upper cladding layer 36 and the cap layer 38 overlying the etch stop layer 34 have a mesa structure. The quantum-well layers 22,26, the barrier layer 24, and optical confinement layers 18,20,28,30 are oftentimes referred to as the active layers.
The chief factor for restricting the increase of the optical output power from the GaAs based ridge waveguide quantum-well laser diode (hereinafter referred to as simply GaAs quantum-well laser diode) is catastrophic optical damage (COD) which signifies that the optical facet of the laser diode is damaged instantaneously. When the optical output power increases to reach a specific value, the COD which is inherent in the GaAs based laser diode is generated to stop the function of the laser diode at that instant.
For preventing the generation of COD failure, for instance, a conventional wide mesa structure of about 4 μm is adopted to decrease the optical density in the active layers; however, in case of the wide mesa structure, a so-called spatial-hole-burning phenomenon occurs wherein the optical gain of the laser diode is uneven in the direction parallel to the axis of the active layers arises to thereby tend to generate a beam steering phenomenon.
The beam steering phenomenon is known in the art and it means that the light beam moves in the direction parallel to the active layers (as viewed from the front facet of the laser), which causes the characteristic of the optical output with respect to injected current in the semiconductor laser diode to be non-linear by having one or more kinks in the characteristic, thereby deteriorating the laser characteristic significantly. The non-linearity means that the external differential quantum efficiency “η” (η=dLOUT/d(I−ITH)) does not remain as a constant, wherein “LOUT” represents the optical output, “I” represents the injected current at the optical output of “LOUT”, and “ITH” represents a threshold current. In a more extreme case, “η” sometimes becomes almost 0. Accordingly, signal conversion cannot be effected from the injected current due to the deteriorated laser characteristic. This is particularly critical when the laser diode is coupled to an optical fiber.
For preventing the generation of the problem beam steering phenomenon in a high output power range, it is necessary to consider complicated and various effects, such as the waveguide mode control of the active layers, and thus the problem is not solved in the conventional GaAs quantum-well laser diode.
In addition, it is important to obtain an operational stability in the laser diode's output at higher output power levels, especially in the stability of the transverse and longitudinal modes of the diode when one uses a GaAs-based quantum-well laser diode as a light source for a WDM system.
As another application, a new optical module wherein a GaAs-based quantum-well laser diode and a fiber Bragg grating (FBG) for controlling the lasing wavelength and gain of the diode are integrated together is now being put to practical use as the pumping light source for erbium-doped fiber amplifiers (EDFAs). However, if a GaAs-based quantum-well laser diode having a lasing wavelength of 980 nm is used as a light source for an EDFA, the longitudinal mode operation is unstable when light returns from the FBG, whereby the optical output power fluctuates to cause another problem.
In the above description for the problems in the ridge waveguide semiconductor laser diodes, a GaAs-based quantum-well laser diode is exemplified. However, these problems are not peculiar to the GaAs-based quantum-well laser diode, but are common to general ridge waveguide semiconductor laser diodes.